The present invention relates generally to a patient ventilation system, and more particularly, to a gas identification (ID) and a volumetrically corrected gas delivery system applied to a ventilation system.
Ventilation treatments have been used as a therapeutic intervention to relieve dyspnea for a patient in acute respiratory distress directly related to severe increases in airway resistance. For moving gas into and out of the lungs, an open airway for the gas to flow from the higher pressure zone to the lower pressure zone is required. The greater the pressure between two points of the airway, the greater volume of gas is moved within the airway. The pressure in airway is directly related to the dynamic pressure gradient during the respiratory cycle, the flow rate of the gas, the density and viscosity of the gas, and the caliber and length of the airway.
Being an inert gas, helium does not participate or interfere with any biochemical process of the body. That is, helium itself has no curative value and cannot support life. However, as helium is the second lightest gas, it is often mixed with oxygen to decrease the gas density, so as to decrease the amount of pressure required for moving gas through the airway. For clinical application, helium typically is mixed with at least 21% oxygen due to its lack of pharmacological properties.
According to specific condition of each patient, the composition of the mixture of helium and oxygen (heliox) is altered. For example, when the patient is in need of a higher concentration of oxygen, a higher driving pressure is required for delivering the gas to the patient. On the contrary, when a lower concentration of oxygen is needed by the patient, the concentration of helium can be increased to dilute the oxygen, and to thereby reduce the required driving pressure of the gas. In order for the therapist/clinician to effectively provide adequate ventilation in obstructive disease patients such as chronic obstructive pulmonary disease or asthma, it is imperative that the clinician understands the relationship between airway resistance and gas characteristics.
Prior art ventilating systems that supply oxygen to a patient normally have two gas inlets, one of which is connected to an oxygen source, and the other is connected to a second gas source. Heliox with 80% of helium and 20% of oxygen, heliox with 70% of helium and 30% of oxygen, or air (with 21% of oxygen and 78% of nitrogen) are frequently applied as the second gas in such ventilating systems. After entering the ventilating system, oxygen and the second gas are mixed in a blender. According to the specific condition of the patient, the fractional inspiratory oxygen flow rate (FIO2) of the gas mixture (oxygen and the second gas) is set by the clinician, and the volumes of oxygen and the second gas entering and exiting the blender are controlled to provide an adequate oxygen flow rate (FIO2) to the patient. However, such adjustment may be inaccurate when the gas composition is altered. For example, when one applies heliox with 80% of helium and 20% of oxygen to a ventilating system of which the blender is driven according to the volumetric flow of heliox with 70% helium and 30% oxygen, the actual FIO2 air or oxygen flow rate is very likely lower than the clinician set value. In addition to obtaining an ineffective ventilation, such inaccuracy may cause a life threatening event, especially when a patient is in critical condition.
Being controlled by the blender, the gas mixture is then delivered to the patient. Again, the exact gas flow rate delivered to the patient is critical. Therefore, a flow sensor is typically installed in the prior art on the inspiratory circuit of the ventilating system to ensure that an adequate rate of gas is flowing to the patient. Similarly, the expiratory flow is also typically monitored and controlled in the prior art via a flow sensor installed in the expiratory flow circuit. To obtain an accurate measurement of the oxygen flow rate, the blender and the flow meters are calibrated before being used. However, the blender and the flow meters incorporated in the conventional prior art ventilating system are calibrated for air instead of the exact gas composition supplied by the ventilating system. This again suppresses the desired oxygen delivery effectiveness of the ventilation system. When the patient is under a critical condition, such delivery ineffectiveness and/or inaccuracy may endanger the patient.
Therefore a substantial need exists in the art to provide a gas ID that can automatically and reliably detect the exact kind of gas supplied to a ventilating system, so as to provide an exact oxygen flow rate required by the patient. Further, correction for calibration of the blender and the flow sensors is also needed to appropriately assist the property prescribed respiration therapy of the patient.